Powdery mildew is a plant disease caused by fungal parasite pathogens. Powdery mildew caused by the fungus Leveillula taurica is one of the most important diseases in pepper, mainly in sweet pepper, and it also affects other vegetable crops. Since the early 1990s this pathogen is reported to cause epidemics in greenhouses and in open fields, in most of the pepper production areas. Whereas most of the fungi species causing powdery mildew are ectoparasites, the mycelium of L. taurica grows intercellularly within the host tissues, limiting the efficiency of chemical fungicides. One of the most pronounced symptoms of the disease caused by this parasite is premature leaf shading, which strongly affects fruit production and quality, rendering significant portion of the fruit unmarketable.
Various resistance sources to powdery mildew were found under natural infection conditions in different pepper (Capsicum) species, including Capsicum annuum, C. baccatum and C. chinense. Quantitative genetic analyses have shown that three to five genetic factors with significant epistatic effects are involved in the resistant phenotype under severe infection conditions (Daubèze et al., 1995. Plant Breed. 114:327-332; Murphy and Deshpande, 1997. Vegetable Sci. 24:127-131). Recently, seven genomic regions including additive quantitative trait locus (QTL) and epistatic interactions were detected in the highly powdery mildew resistant C. annuum genitor H3, which has small, pungent fruit. These findings, i.e., the complex inheritance pattern of resistance together with the observation that powdery mildew epidemic is highly dependent on environmental conditions, explain the high variability found in the response of pepper plants to the pathogen (Lefebvre et al., 2003. Theor. Appl. Genet. 107:661-666).
U.S. Pat. No. 6,350,933 discloses nucleic acids and proteins which confer powdery mildew disease resistance. The nucleic acids can be used to produce transgenic plants of various types that are resistant to pests. Antibodies to proteins disclosed in the invention are also provided.
U.S. Pat. No. 6,677,510 discloses plants resistant to powdery mildew, specifically dogwood (Cornus florida) cultivars that are resistant to infestation with powdery mildew fungi, and materials and methods for identifying, characterizing, and/or producing powdery mildew resistant plants. It also discloses polynucleotide sequences, and patterns of polynucleotide sequences, which are associated with resistance to powdery mildew, useful in identifying and characterizing plants having resistance to powdery mildew. However, these means and methods are mainly directed to ornamental plants.
Plant viruses are a continuing problem in the agricultural industry. Viral infection in plants causes a variety of undesirable effects including stunted growth, altered morphology, reduced yield, diminished quality and increased susceptibility to damage by other pests. Attempts to control or prevent infection of a crop by a plant virus have been made, yet viral pathogens continue to be a significant problem in agriculture.
Potyviruses comprise about 30% of all known plant viruses, and viruses belonging to this group cause significant damage to agricultural crops. The family Potyviridae is characterized by a single-stranded sense RNA genome with a covalently bound viral-encoded protein (VPg) attached to the 5′ terminus, and a 3′ poly-A tract. The genome is approximately 10 kb in length and is translated as a polyprotein that is subsequently cleaved into smaller polypeptides by viral-encoded proteases. Potato virus Y (hereinafter PVY) is a member of the potyvirus plant virus group. PVY is a positive-sense, single-stranded RNA virus that is surrounded by a repeating proteinaceous monomer, which is termed the coat protein (CP). The encapsidated virus has flexous rod morphology, which is characteristic of the potyvirus group. The majority of the potyviruses, including PVY, are transmitted in a non-persistent manner by aphids. The host range of PVY includes potato, tobacco, tomato and pepper, and the ease of its transmittance by aphids results in a significant damage to these crops and to a drastic reduction in their economic value.
Potyvirus infection requires the interaction of host factors with viral proteins and RNA for viral replication and systemic spread. The “negative model” of plant virus resistance predicts that a recessive resistance gene may represent a deleted or defective host protein that is essential for viral infection but is not necessary for the normal function of the plant cell. Recessive resistance is especially prevalent for potyviruses, comprising approximately 40% of all known resistance genes. Several host genes whose mutations impair the infection cycle of plant viruses have been characterized, particularly in Arabidopsis (Kang et al. Annu. Rev. Phytopathol. 43:581-621). The gene encoding the translation initiation factor eIF4E has been identified repeatedly in diverse hosts as a naturally occurring recessively inherited resistance locus. Mutations in this gene have been shown to produce resistance to potyviruses in several plant species including pepper. The mutations occur at the pvr1 locus, recently shown to be identical to the pvr2 locus (Ruffel et al., 2002. The Plant J. 32:1067-1075; Kang et al., 2005. The Plant J. 42:392-405).
Many of the genes involved in virus resistance, including the virus resistance gene pvr1 identified in Capsicum, have been used successfully as effective and stable markers for identifying sources of resistance in crop breeding programs. Yearn et al. (2005. Theor. Appl. Genet. 112:178-186) reported the generation and use of molecular markers that define loci for selection of potyvirus-resistance Capsicum plants, using cleaved amplified polymorphic sequences (CAPS). Such CAPS markers were developed for three recessive viral resistance alleles, pvr1, pvr11 and pvr12.
Alternatively, attempts are directed towards the use of genetic engineering techniques to produce virus resistance plants (for example, U.S. Pat. Nos. 5,589,612; 5,939,603; 5,986,175; and 6,806,400).
As of today, pepper plants resistant to potyviruses, including varieties suitable for commercial crop growth are available. Nevertheless, chemical control is still widely used to combat viral infection.
In recent decades, awareness of the harmful effect of extensive use of pesticides on the environment has been constantly grown. Efforts are directed at the development of sustainable agriculture, including maintaining high yield crop production with significant reduction in employing environmentally hazardous chemicals.
Chemical control of the fungus L. taurica, causing powdery mildew disease in pepper cultivars, involves high costs and is hazardous to the environment.
Thus, there is a great demand for, and it would be highly advantages to have pepper plants, suitable for growth on a commercial scale, which are resistant to more than one pathogen.